Method of Converting Signals For Multi-Primary Color Display

ABSTRACT

A method is described which converts a three primary color input signal (Cx, Cy, Cz) into N drive signals (P 1 , . . . , PN) to drive N&gt;3 primary colors of a multi-primary color display ( 3 ). The method determines ( 1 ) a valid range (VS) wherein the N drive signals (P 1 , . . . , PN) have valid values by performing the steps of (i) defining 3 functions representing 3 of the drive signals (P 1 , P 2 , P 3 ) as a function of the remaining N−3 drive signal(s) (P 4 , . . . , PN), and (ii) calculating ( 1 ) a common range in a space formed by the N−3 drive signal(s) (P 4 , . . . , PN) wherein each one of the 3 functions has valid values. The method selects ( 2 ) a point from the common range to determine the N drive signals (P 1 , . . . , PN).

FIELD OF THE INVENTION

The invention relates to a method of converting a three primary colorinput signal into N drive signals for driving N>3 primary colors of amulti-primary color display. The invention further relates to a systemfor converting a three primary color input signal into N drive signals,and to a display apparatus comprising this system.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,570,584 discloses an OLED display device with more thanthe conventional red, green, and blue sub-pixels. The additionalsub-pixel may have a color selected outside the color gamut possiblewith the red, green, and blue sub-pixels to obtain a wider color gamut.The display system uses a conventional RGB input signal to create anappropriate multi-color signal for driving all the sub-pixels.Alternatively, the additional sub-pixel may have a color inside thecolor gamut possible with the red, green, and blue sub-pixels toincrease the efficiency and lifetime of the display.

A color data transformation circuit converts the input signal to valuesnecessary for controlling each of the sub-pixels. The transformationcircuit calculates the appropriate amounts of light from each of thethree available sub-pixel colors to reproduce the desired color at eachof the four sub-pixels using well known matrix transforms or lookuptables. The amount of light produced by a sub-pixel depends on anumerical value supplied to the sub-pixel. The numerical value is, forexample an eight or six bit value. In general, many differentpossibilities exist to transform the input signal to the drive values ofthe sub-pixels.

A transformation circuit which performs matrix transformations has thedrawback that a lot of matrix multiplications are required. Either anexpensive and fast calculation circuit is required or an algorithm hasto be constructed to pre-calculate the majority of the computationallyintensive mathematical calculations to enable real time operation. Apre-calculation and a transformation circuit which uses look-up tableshave the drawback that the algorithm is pre-fixed to one of the possiblesolutions.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of converting athree primary color input signal into N drive signals for driving N>3primary colors of a multi-primary color display which has a highflexibility in selecting out of the possible solutions.

A first aspect of the invention provides a method as claimed in claim 1.A second aspect of the invention provides a system as claimed in claim10. A third aspect of the invention provides a display apparatus asclaimed in claim 11. Advantageous embodiments are defined in thedependent claims.

The method in accordance with the first aspect of the invention convertsa three primary color input signal into N drive signals which drive N>3primary colors of a multi-primary color display. The three primary colorinput signal usually is a RGB signal, but any other signal which definesthe drive signals of a display which has pixels with three differentlycolored sub-pixels may be used instead. The N drive signals are suppliedto the more than 3 sub-pixels of the multi-primary color display.Usually, the multi-primary color display has, per pixel, the threestandard RGB sub-pixels and at least one further sub-pixel with adifferent color. The different color may be selected inside or outsidethe color gamut defined by the RGB sub-pixels. The different color maybe white.

The method determines a valid range wherein the N drive values havevalid values. Three functions are defined which represent 3 of the drivesignals as a function of the remaining N−3 drive signal(s). The validranges of the N−3 drive signal(s) are calculated taking the valid rangesof the 3 drive signals into account. A valid range of drive values isthe range to which the values of the drive values are restricted.Usually, the valid range is determined by the number of bits availablefor the values. For example, if 8 bits words are used, the valid rangeranges from 0 to 255, including the border values. Alternatively, in ananalog implementation, the valid range usually is limited by a minimumand maximum possible voltage. To ease the calculations, often anormalized valid range is used which ranges from 0 to 1, including theborder values, and which is independent of the actual digital or analogdrive range. The last “s” of the N−3 drive signal(s) is put betweenbrackets to indicate that if N=4 only one remaining drive signal ispresent, while if N>4 a plurality of remaining drive signals is present.

The valid drive values of the N−3 drive signal(s) are determined bytaking into account valid drive ranges of the 3 drive signals. Each oneof the 3 functions is defined by values formed a space together with theN−3 drive signal(s). Thus, for each one of the 3 functions is determinedin which range of the space formed by the N−3 drive signal(s) thefunctions have valid values. The common part in the space formed by theN−3 drive signals of the valid ranges found for the 3 drive signals isthe valid range of the N−3 drive signal(s). This valid range of the N−3drive signal(s) is a line part if N=4, a two-dimensional area if N=5, athree-dimensional volume if N=6, or a multi-dimensional volume if N>6.The valid range directly indicates which possible selections of the N−3drive signals are possible in the conversion.

Once the valid range of the N−3 drive signal(s) is calculated, a pointout of this range is selected. The selected point immediately definesthe values of the N−3 drive signal(s). The values of the remaining 3drive signals are determined by substituting the selected values of theN−3 drive signal(s) in the 3 functions.

This approach does not require calculating the N drive signals withmatrix operations or with look up tables. Instead, only theintersections of the 3 functions with the borders of the valid ranges ofthese 3 functions have to be calculated. These intersections providevalues in the space defined by the N−3 drive signal(s) which are used todetermine the common area. Thus, reasonably simple operations which canbe performed in real-time suffice. If N=4, the 3 functions defining thethree drive signals as a function of the fourth drive signal arestraight lines. Now, only intersections of these lines with the borderlines indicating the borders of the valid values of the respective drivesignals have to be determined. If N=5, the 3 functions defining thethree drive signals as a function of the fourth and the fifth drivesignals are planes. Now, only intersections of these planes with theborder planes defining the border values have to be calculated todetermine the resultant border lines in the space defined by the fourthand the fifth drive signal. This approach can be extended to N>5.

Once this valid common range in the N−3 space is determined, thealgorithm has the flexibility to select a particular point out of thisrange. This provides the flexibility to adapt, in real time, theselection to changing situations.

In an embodiment in accordance with the invention as defined in claim 2,each of the three functions is defined as a sum of one of the colors ofthe input signal and a linear combination of the N−3 drive signals(s).The weighting factors of the linear combination are coefficients definedby the color points of the N sub-pixels. Thus, if the color points donot vary in time or with temperature, the coefficients are predeterminedfixed values.

In an embodiment as defined in claim 3, the selection of the point inthe common range is performed under a constraint for at least one aspectof the 3 drive signals. Due to the relatively low computational effortrequired to determine the common range of drive values, a real timecalculation is possible. When the common range has been determined, itis possible to select an appropriate point from this range to calculatethe N drive signals. This selection can be performed under a constraintwhich is fixed on beforehand, or which is determined in real-time. Thefirst approach has the advantage that a real-time algorithm can be usedwithout requiring a complex and expensive converter. The second approachhas the advantage that the real-time algorithm has a high flexibilitybecause it is possible to cater in real-time for changes of theconstraint.

In an embodiment as defined in claim 4, the point is selected from thecommon range such that the value of one of the drive signals is minimal.The minimal drive of this particular one of the drive signals allowsmaximizing its lifetime. This is particularly interesting if thesub-pixels associated with this particular drive signal have a shorterlifetime than the other sub-pixels.

In an embodiment as defined in claim 5, the point is selected from thecommon range such that a real-time varying constraint is met. Theconstraint may be determined by the temperature of the display, anamount of ambient light impinging on the display, aging of the display,or a power consumption of the display.

In an embodiment as defined in claim 6, preferred functions are definedif four drive voltages are required to drive pixels which have foursub-pixels.

In an embodiment as defined in claim 7, because the 3 functions definethe 3 drive values as a linear function of the fourth drive value, thecommon range is a line segment on the line indicating values of thefourth drive value. The common range is the overlap of the valid rangesof the 3 functions on the line indicating the values of the fourth drivevalue.

In an embodiment as defined in claim 8, the valid range on the lineindicating the values of the fourth drive value is determined for eachone of the 3 functions by calculating the values of the fourth drivevalue where each one of the 3 functions reaches the borders of the validrange of their values.

In an embodiment as defined in claim 9, the drive ranges are normalizedsuch that the border values are zero and one, respectively. Thissimplifies the algorithm in that it is not required to adapt thealgorithm to the actual ranges used. The actual ranges usually depend onthe particular application. Further, the determination of the fourthdrive values where each one of the 3 functions reaches the borders ofthe valid range is simplified to solving the equations for the valueszero and one, respectively, of the functions.

The system of claim 10 may be a system realized by hardware or bysoftware or a combination thereof. The hardware may be included in anintegrated circuit or may be a combination of electronic components.

The system may also be a computer program product comprising a program 5for performing the steps as claimed in claim 1.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematical block diagram of a display device comprisinga converter in accordance with the invention,

FIG. 2 shows an example of the determination of the common range in aconverter which has to convert the three color input signal into fourdrive signals for four sub-pixels,

FIG. 3 shows a more detailed block diagram of a converter for a three tofour primary conversion using static and/or dynamic constraints, and

FIG. 4 shows an example for elucidating the determination of the commonrange in a converter which has to convert the three color input signalinto five drive signals for five sub-pixels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a schematical block diagram of a display device comprisinga converter in accordance with the invention. The display devicecomprises a signal processing circuit 4, the converter, and a display 3.

The converter comprises a valid range determining function 1, furtherreferred to as the range function 1, a selection function 2, and adisplay drive function 5.

The signal processing circuit 4 receives an image input signal IV andsupplies the primary color input signal Cx, Cy, Cz. The range function 1receives the three input signals Cx, Cy, Cz and a coefficient matrix MCto supply the common range VS of the N−3 drive values P4 to PN. Thethree input signals Cx, Cy, Cz are commonly referred to as IS, and areusually the RGB signals, but may be other signals defining the amount oflight of the three sub-pixels per pixel of a standard three sub-pixeldisplay. In accordance with the present invention, these three inputsignals IS have to be transformed into, or mapped on, drive signals P1to PN for the N>3 sub-pixels 30 to 33 per pixel 34 of the more thanthree sub-pixel display 3. In FIG. 1, by way of example, each pixel 34comprises four sub-pixels 30 to 33. The drive signals P1 to PN arecollectively also referred to as DS. The range function 1 uses threefunctions which each define one of the three drive values P1 to P3 as afunction of the input signals IS and the remaining N−3 drive values P4to PN. It has to be noted that the remaining drive values P4 to PN maybe a single fourth drive value if N=4. The coefficient matrix MC isdefined by the color points of the N sub-pixels.

The operation of the range function 1 is detailed for normalized drivevalues with respect to the example for the three to four primaryconversion (N=4) in FIG. 2, and the example for the three to fiveprimary conversion (N=5) in FIG. 3. From the elucidation of theseexamples it is clear how the range function 1 has to operate for N>5.

The selection function 2 receives the valid range VS, the input signalsIS, the coefficient matrix MC, and an optional selection criterion orconstraint CON to supply the N drive signals DS via the display drivefunction 5 to the sub-pixels 30 to 33 of the display 3. The displaydrive function 5 may comprise a gamma function when the operations infront of the display drive function 5 are performed in the linear lightdomain.

FIG. 2 shows an example of the determination of the common range in aconverter which has to convert the three color input signal into fourdrive signals for four sub-pixels. The fourth drive signal P4 isdepicted along the horizontal axis, and the three drive signals P1 to P3along the vertical axis. The three drive signals P1 to P3 are defined asfunctions P1(P4) to P3(P4) of the fourth drive signal P4. The commonrange VS of the fourth drive signal P4 in which all the three drivesignals P1 to P3 have values within their valid ranges runs from thevalue P4,min to P4,max.

Preferably, the functions defining the three drive signals P1 to P3 aredefined by

$\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{{k\; 1},1} & \ldots & {{k\; 1},{N - 3}} \\{{k\; 2},1} & \ldots & {{k\; 2},{N - 3}} \\{{k\; 3},1} & \ldots & {{k\; 3},{N - 3}}\end{bmatrix} \times \begin{bmatrix}{P\; 4} \\\ldots \\{PN}\end{bmatrix}}}$

wherein P1 to PN are the N drive signals, (P1′, P2′, P3′) are defined bythe input signal IS, and the coefficients ki,j of the matrix MC define adependence between 3 primaries associated with the 3 drive values P1 toP3, and the N−3 other primaries associated with the N−3 drive signal(s)P4 to PN. For N=3, P1′, P2′, P3′ are identical to P1, P2, P3,respectively.

To further elucidate the relation between the elements of thesefunctions it is now shown for the example of a three to four primaryconversion how the above functions relate to the standard three to fourprimary conversion. In a standard three to four primary conversion, thedrive signal DS, which comprises the drive signals P1 to P4, istransformed to the linear color space XYZ by the following matrixoperation.

$\begin{matrix}{\begin{bmatrix}{Cx} \\{Cy} \\{Cz}\end{bmatrix} = {\begin{bmatrix}{t\; 11} & {t\; 12} & {t\; 13} & {t\; 14} \\{t\; 21} & {t\; 22} & {t\; 23} & {t\; 24} \\{t\; 31} & {t\; 32} & {t\; 33} & {t\; 34}\end{bmatrix} \times \begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3} \\{P\; 4}\end{bmatrix}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The matrix with the coefficients tij defines the color coordinates ofthe four primaries of the four sub-pixels. The drive signals P1 to P4are unknowns which have to be determined by the multi-primaryconversion. This equation 1 cannot be solved immediately because thereare multiple possible solutions as a result of introducing the fourthprimary.

The question solved in the present invention is how to make an algorithmthat efficiently deals with the freedom in conversion and which isefficient such that a real-time determination of the common range ispossible. Preferably, also a real-time selection out of thesepossibilities for the drive values of the drive signals P1 to P4 ispossible. It is known to use particular constraints applied to thealgorithm to reduce or even eliminate the freedom of selection. However,these constraints are “hard-embedded” in the algorithm and cannot bechanged in response to the determined possible drive values such that areal-time varying constraint determines the selection.

Equation 1 can be rewritten into:

$\begin{matrix}{\begin{bmatrix}{Cx} \\{Cy} \\{Cz}\end{bmatrix} = {{{\lbrack A\rbrack \times \begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix}} + {\begin{bmatrix}{t\; 14} \\{t\; 24} \\{t\; 34}\end{bmatrix} \times P\; 4\mspace{14mu} A}} = \begin{bmatrix}{t\; 11} & {t\; 12} & {t\; 13} \\{t\; 21} & {t\; 22} & {t\; 23} \\{t\; 31} & {t\; 32} & {t\; 33}\end{bmatrix}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

wherein the matrix [A] is defined as the transforming matrix in thestandard three primary system. Multiplication of the terms of equation 2with the inverse matrix [A⁻¹] provides Equation 3.

$\begin{matrix}{\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} = {\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} + {\left\lbrack A^{- 1} \right\rbrack \times \begin{bmatrix}{t\; 14} \\{t\; 24} \\{t\; 34}\end{bmatrix} \times P\; 4}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The vector [P1′ P2′ P3′] represents primary values as they would be ifthe display system only contains three primaries. Finally, Equation 3 isrewritten into Equation 4.

$\begin{matrix}{\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{k\; 1} \\{k\; 2} \\{k\; 3}\end{bmatrix} \times P\; 4}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Thus, the driving signal of any three primaries P1 to P3 is expressed byEquation 4 as a function of the fourth primary P4. The coefficients k1to k3 are equal to the inverse matrix A⁻¹ multiplied by the respectivecoefficients t14 to t34. These linear functions define three lines in a2 dimensional space defined by the fourth primary P4 and the values ofthe fourth primary P4 as is illustrated in FIG. 2. All values in FIG. 2are normalized which means that the values of the four drive values P1to P4 have to be within the range 0≦Pi≦1, with Pi being one of the drivevalues P1 to P4. From FIG. 2 it directly becomes clear visually what thecommon range VS of P4 is for which all the functions P1 to P3 havevalues which are in the valid range.

Basically, if no such common range VS exists, then the input color isoutside the four primary color gamut and thus cannot be correctlyreproduced. For such colors a clipping algorithm should be applied whichclips these colors to the gamut.

A scheme which calculates the common range P4,min to P4,max iselucidated with respect to FIG. 3.

FIG. 3 shows a more detailed block diagram of a converter for a three tofour primary conversion using static and/or dynamic constraints.

As is clear from FIG. 2, the boundary values P4,min and P4,max of thecommon range VS of the drive signal P4 may be determined as elucidatedin the now following. Firstly, for each one of the 3 drive signals P1 toP3 a minimum boundary value and a maximum boundary value is determined:

if ki>0, and the line defining a particular drive signal Pi isinclining,

a minimum boundary value of the valid range for this Pi is: −Pi′/ki,

ki is one of the coefficients k1 to k3,

a maximum boundary value of the valid range for this Pi is: (1−Pi′)/ki

if ki<0, and the line defining a particular drive signal P isdescending,

a maximum boundary of the valid range for this Pi is: −Pi′/ki

a minimum boundary of the valid range for this Pi is: (1−Pi′)/ki

Secondly, also the values of the drive signal P4 have to be within thevalid range of values:

0≦P4≦1.

Thirdly, the common range VS of values of the drive signal P4 isdetermined by selecting the maximum value of the minimum boundary valuesand the minimum value of the maximum boundary values. This is nowfurther elucidated by describing the block diagram shown in FIG. 3 indetail.

An inverter 40 receives the signal P1′ to supply the negative value −P1′of the signal P1′. An inverter 41 receives the signal P2′ to supply thenegative value −P2′ of the signal P2′. An inverter 42 receives thesignal P3′ to supply the negative value −P3′ the signal P3′.

A multiplier 43 receives the negative value −P1′ and the coefficient1/k1 to supply the signal −P1′/k1. A multiplier 44 receives the negativevalue −P2′ and the coefficient 1/k2 to supply the signal −P2′/k2. Amultiplier 45 receives the negative value −P3′ and the coefficient 1/k3to supply the signal −P3′/k3. The coefficients 1/k1, 1/k2, 1/k3 or k1,k2, k3 may be stored in a memory 58.

An adder 46 sums the coefficient 1/k1 to the signal −P1′/k1 to obtainthe signal (1−P1′)/k1. An adder 47 sums the coefficient 1/k2 to thesignal −P2′/k2 to obtain the signal (1−P2′)/k2. An adder 48 sums thecoefficient 1/k3 to the signal −P3′/k3 to obtain the signal (1−P3′)/k3.

A sign circuit 49 receives the coefficient 1/k1 or the coefficient k1 togenerate a sign signal SI1 which indicates whether the coefficient 1/k1or k1 is positive or negative. A sign circuit 50 receives thecoefficient 1/k2 or k2 to generate a sign signal SI2 which indicateswhether the coefficient 1/k2 or k2 is positive or negative. A signcircuit 51 receives the coefficient 1/k3 or k3 to generate a sign signalSI3 which indicates whether the coefficient 1/k3 or k3 is positive ornegative. A positive coefficient indicates that the drive signal P1, P2,P3 as function of the drive value P4 is an inclining line, and anegative coefficient indicates that the drive signal P1, P2, P3 asfunction of the drive value P4 is a descending line.

A switch circuit 52 receives the sign signal SI1, the signal −P1′/k1,and the signal (1−P1′)/k1. If the sign signal SI1 indicates that 1/k1 ispositive, the signal −P1′/k1 is supplied to the circuit 55, and thesignal (1−P1′)/k1 is supplied to the circuit 56. If the sign signal SIindicates that 1/k1 is negative, the signal (1−P1′)/k1 is supplied tothe circuit 55, and the signal P1′/k1 is supplied to the circuit 56.

The circuit 55 determines the maximum value MAV of all the input valuesreceived for the three drive signals P1 to P3 to determine the leftborder value P4,min of the common range on the axis P4. The circuit 56determines the minimum value MIV of all the input values received forthe three drive signals P1 to P3 to determine the right border valueP4,max of the common range on the axis P4. The select circuit 57receives the values P1′ to P3′, the coefficients 1/k1 to 1/k3 or k1 tok3, and optional constraints CON to calculate the drive signals P1 to P4in accordance with Equation 4. The select circuit 57 selects out of thecommon range VS which is defined as the values of P4 which are in therange P4,min to P4,max, including the border values, the actual value ofP4 which will be used to calculate the drive values of the drive signalsP1 to P3 by substitution of the actual value of P4 in the Equation 4.The selection of the actual value of P4 may depend on the constraintsCON.

By applying additional constraints CON to the determined possiblemapping interval VS (the common values of P4) an optimal solution can beselected fitting the constraints CON. This selection can be performed onsoftware level which is easily adaptable. As is shown, by way ofexample, for the three to four primary conversion in FIG. 3, thealgorithm used can be performed with a simple and efficientimplementation in hardware or software. No costly look-up tables or highintensity calculations are required. Further, the algorithm is able toadapt to static or even dynamic constraints CON.

An example of such a static constraint is to select a point from thecommon range VS such that a particular primary P1 to P4 has a minimumdrive value. This algorithm is particularly simple if the primary whichshould have the minimum drive value is the drive signal P4. For example,in a multi-primary OLED display, in which typically the blue primary hasthe shortest lifetime, the drive signal for the blue OLED is selected tobe the drive signal P4. Now, the minimum value P4,min of the commonrange is selected and the lifetime of the blue OLED is maximized whilestill the correct color is displayed.

Most interesting is the possibility to adapt the selection to dynamicconstraints CON. This allows a real-time adaptation of the multi-primaryconversion to, for example, temperature, ambient lighting, powerconsumption or display aging. Such a real-time adaptation is notpossible in prior art algorithms with fixed build in constraints. Areal-time adaptation is for example especially relevant if the primaryintensities vary with time.

Efficient constrained clipping could require both static and dynamicconstraints. Typically, color clipping is not a trivial task formulti-primary applications, since it has to efficiently deal with colorswhich are outside the display gamut. The clipped value should beperceived as similar to the input color as possible. This similarity isdefined by different perception constraints, which can be eitherobjective or subjective. Knowing the dependence between the primariesand using perception constraints, it is possible to determine theoptimum clipped value.

Thus, the proposed algorithm can be efficiently used in any applicationwhich requires a multi-primary conversion, for example, in amulti-primary monitor or television display, or mobile applications suchas, for example, a mobile phone or a PDA.

FIG. 4 shows an example for elucidating the determination of the commonrange in a converter which has to convert the three color input signalinto five drive signals for five sub-pixels. If the three input signalshave to be converted into five drive signals, the three functions whichdefine the relation between the three drive signals P1 to P3 and theother two P4, P5 are planes in the three-dimensional space formed by thetwo drive signals P4, P5 and the values of the three functions. Just forsimplicity, only one of the planes defined by one of the three functionsis shown. The function shown is the plane defining the values of thedrive signal P1 as a function P1(P4, P5) of the drive signals P4 and P5.The valid range of the two drive signals P4, P5 due to the valid rangeof this function is the hatched area A1 in the plane defined by thesetwo drive signals P4, P5. This hatched area A1 is obtained bydetermining the valid range of the function defining the drive signalP1. This valid range A1 of the drive signal P1 is bounded by theintersection lines L1 and L2 of the plane where the values of the drivesignal P1 are the border values zero and one, respectively. The line L1already is present in the plane formed by the drive signals P4, P5, andthe line L2 has to be projected on the plane defined by the drivesignals P4, P5 to obtain the line L2′. The area between the lines L1 andL2′ bounded by the border values zero and one of the drive signals P4,P5 are the valid range of the drive signals P4, P5 caused by the validrange of the drive signals P1. In a same way, but for reasons of claritynot shown in FIG. 4, the valid ranges (areas in the P4, P5 plane) of thedrive signals P4, P5 can be obtained for the drive signals P2 and P3.The common, overlapping part of these three valid ranges in the P4, P5plane defines the common rang VS from which the drive values of thedrive signals P4 and P5 can be selected. Once a point is selected in thecommon range VS, the other drive signals P1 to P3 can be calculated bysubstituting the values of P4 and P5 of the point selected in the threefunctions. It has to be noted that the common range VS is: a range on aline if N=4 (see FIG. 2), an area in a plane if N=5 (see FIG. 4) or athree or more dimensional volume if N>5. For N>5 the determination ofthe common range VS is performed by first determining for each of thethree functions the borders of their valid ranges in the space definedby the N−3 remaining drive values.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. A method of converting a three primary color input signal (Cx, Cy,Cz) into N drive signals (P1, . . . , PN) for driving N>3 primary colorsof a multi-primary color display (3), the method comprises: determining(1) a common range (VS) wherein the N drive signals (P1, . . . , PN)have valid values by performing the steps of (i) defining 3 functionsrepresenting 3 of the drive signals (P1, P2, P3) as a function of theremaining N−3 drive signal(s) (P4, . . . , PN), and (ii) calculating (1)the common range (VS) in a space formed by the N−3 drive signal(s) (P4,. . . , PN) wherein each one of the 3 functions has valid values, andselecting (2) a point from the common range (VS) to determine the Ndrive signals (P1, . . . , PN).
 2. A method as claimed in claim 1,wherein the 3 functions are defined by $\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{{k\; 1},1} & \ldots & {{k\; 1},{N - 3}} \\{{k\; 2},1} & \ldots & {{k\; 2},{N - 3}} \\{{k\; 3},1} & \ldots & {{k\; 3},{N - 3}}\end{bmatrix} \times \begin{bmatrix}{P\; 4} \\\ldots \\{PN}\end{bmatrix}}}$ wherein P1 to PN are the N drive signals, (P1′, P2′,P3′) are defined by the input color, and the coefficients ki,j define adependence between 3 primaries associated with the 3 drive values P1,P2, P3, and the other primaries associated with the N−3 drive signal(s)P4 to PN.
 3. A method as claimed in claim 1, wherein the selecting (2)is performed under a constraint (CON) for at least one aspect of atleast one of the 3 drive signals (P1, P2, P3).
 4. A method as claimed inclaim 3, wherein the constraint (CON) is that one of the drive signals(P1, P2, P3, P4) is selected to have a minimum possible drive value. 5.A method as claimed in claim 3, wherein the constraint (CON) is a realtime constraint determined by at least one of the group of: atemperature of the display (3), an amount of ambient light at thedisplay (3), aging of the display (3), or a power consumption of thedisplay (3).
 6. A method as claimed in claim 1, wherein N=4, and whereinthe 3 functions are defined by $\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{k\; 1} \\{k\; 2} \\{k\; 3}\end{bmatrix} \times P\; 4}}$ wherein P1 to P4 are the 4 drivesignals, (P1′, P2′, P3′) are defined by the input color, and thecoefficients ki define a dependence between 3 primaries associated withthe 3 drive values P1, P2, P3, and the other primary associated with thefourth drive signal P4.
 7. A method as claimed in claim 6, wherein thecalculating calculates the common range (VS) on a line formed by thefourth drive signal (P4), and wherein the common range (VS) defines arange wherein each one of the 3 functions has valid values.
 8. A methodas claimed in claim 7, wherein the calculating of the common range (VS)comprises: calculating, for each one of the 3 functions, a lower bordervalue of the fourth drive signal (P4) where the particular functionreaches one of the border values of the valid range, and a higher bordervalue of the fourth drive signal (P4) where the particular functionreaches another border value of the valid range, and determining thecommon range (VS) from the respective lower and higher border values(P4,min, P4,max).
 9. A method as claimed in claim 1, wherein all drivevalues (P1 to PN) are normalized and wherein valid ranges are defined asthe values in the range from zero to one, including its borders.
 10. Asystem for converting a three primary color input signal (Cx, Cy, Cz)into N drive signals (P1, . . . , PN) for driving N>3 primary colors ofa multi-primary color display (3), the system comprises: means fordetermining (1) a common range (VS) wherein all the N drive signals (P1,. . . , PN) have valid values comprising means for defining 3 functionsrepresenting 3 of the drive signals (P1, P2, P3) as a function of theremaining N−3 drive signal(s) (P4, . . . , PN), and means forcalculating (1) the common range (VS) in a space formed by the N−3 drivesignal(s) (P4, . . . , PN) wherein each one of the 3 functions has validvalues, and means for selecting (2) a point from the common range (VS)to determine the N drive signals (P1, . . . , PN).
 11. A displayapparatus comprising a signal processing circuit (4) for receiving animage input signal (IV) to supply the primary color input signal (Cx,Cy, Cz), the system as claimed in claim 10, and a display device (3) forreceiving the N drive signals (P1 to PN).